Conventionally, engine oil is used to lubricate and cool the engine. The engine oil is stored in an oil pan disposed under the engine and is circulated through individual parts of the engine by an oil pump. The engine oil circulated through the individual parts drops into the oil pan located below these parts. The engine oil dropped into the oil pan is recirculated through the individual parts by the oil pump. During the circulation, the engine oil receives heat from the individual parts of the engine and cools them. The engine oil also acts to form oil films in the individual parts of the engine, thereby promoting lubrications among the parts, preventing the parts from oxidizing, and so forth.
Immediately after the engine is started in a cold state, the engine oil stored in the oil pan is cold and has a high viscosity, so that the engine oil is not in a state suitable for circulating through the individual parts of the engine and lubricating them. It is thus desired to raise the temperature of the engine oil as soon as possible immediately after the cold start and to have an appropriate viscosity. To this aim, it has been proposed to divide an oil pan into multiple sections, so as to prepare a state where the engine oil within one of the sections is likely to circulate immediately after the cold start, and heat the engine oil within this section earlier, while to prevent the engine oil from being excessively heated after the completion of warming up and place the engine oil in a favorable state (See Documents 1 through 3 identified later). The early temperature rise of the engine oil contributes improvements in fuel economy due to early reduction in friction, and is desired in terms of recent strong demands for fuel economy.
FIG. 1 is a cross-sectional view of a dual-chamber type oil pan 50 disclosed in Document 1 (Japanese Patent Application Publication 2003-222012). The dual-chamber oil pan 50 has an oil pan separator 51 having a recess portion 51a in an oil pan 52 in order to efficiently raise the temperature of engine oil. An oil strainer 53 is arranged so that a port 53a for sucking the engine oil is positioned in the recess portion 51a. Communication holes 54 and 55 are respectively provided in upper and lower portions of a sidewall 51a1 of the recess portion 51a so that the inside and outside of the sidewall 51a1 of the recess portion 51a can communicate with each other. The communication hole 55, which is provided in the lower portion of the sidewall 51a1 of the recess portion 51a, controls circulation of the engine oil through the sidewall 51a1 of the recess portion 51a by utilizing variations in the viscosity of the engine oil. More specifically, the communication hole 55 is designed to have a small diameter, which functions as a high circulation resistance for the engine oil having a high viscosity when the engine is in the warmed-up state. It is thus impossible to mix the engine oils located inside and outside of the sidewall 51a1 with each other through the communication hole 55. In contrast, the engine oil having a low viscosity after warming up can pass through the communication hole 55, so that the engine oils located inside and outside of the sidewall 51a1 of the recess portion 51a can be mixed with each other. This mixing causes the engine oil having a low temperature positioned outside of the recess portion 51a to cool the engine oil in the recess portion 51a having a high temperature.
The communication hole 54, which is provided in the upper portion of the sidewall 51a1 of the recess portion 51a, is capable of circulating the engine oil between the inside and outside of the sidewall 51a1 irrespective of the viscosity of the engine oil. The communication hole 54 mainly functions to flow the engine oil that has been circulated through the individual parts of the engine and dropped into the oil pan separator 51 (within the recess portion 51a) to the outside of the sidewall 51a1. Thus, a circulation route of the engine oil indicated by arrows 57 can be formed in which the engine oil flowing out of the upper portion of the recess portion 51a flows in the recess portion 51a again through the lower portion of the recess portion 51a on the basis of the viscosity of the engine oil. The circulation route of the engine oil facilitates mixing and cooling of the engine oil. The mixed engine oil is sucked from the suction port 53a, and is supplied to the inside of the engine block 56. A drain plug 58 is attached to the oil pan 52.
Document 2 (Japanese Patent Application Publication No. 2003-278519) discloses an oil pan structure in which the inside of an oil pan is divided into two oil reservoirs by a separate plate. The upper end of the separate plate is located so as to be lower than the oil level. The separate plate has a communication passage for making a communication between the two reservoirs, and a valve for opening and closing the communication passage in accordance with variations in the temperature of the oil in the oil pan. In the above oil pan structure, only one of the two oil reservoirs is equipped with the suction port of an oil pipe, and only oil in the oil reservoir associated with the suction port is used when the oil is at a low temperature. It is thus possible to quickly raise the temperature of the oil in the oil pan. When the oil temperature rises and the valve is brought in the open state, the two oil reservoirs are allowed to communicate with each other, and the oils in the oil reservoirs are circulated through the individual parts of the engine. The two oil reservoirs always communicate with each other above the top end of the separate plate, and are kept at an identical level.
Document 3 (Japanese Patent Application Publication No. 2001-152825) discloses an oil pan of the engine, which is divided into first and second oil reservoirs by a segment plate. A vertical sidewall of the segment plate has a communication hole via which the first and second oil reservoirs communicate with each other. A first valve is provided which releases the communication hole when the amount of oil in the first reservoir becomes lower than a given level. A second valve is provided which releases the communication hole when the temperature of the oil in the first reservoir becomes higher than a given temperature. The end of the oil strainer, that is, the suction port is located in the first oil reservoir. When the temperature of the engine oil in the first reservoir is low, this oil is used for circulation. It is thus possible to facilitate the temperature rising of a small amount oil in the first oil reservoir. When the amount of the oil in the first oil reservoir becomes lower than the given level, the first and second oil reservoirs are caused to communicate with each other, so that oil shortage can be avoided.